U.S. Pat. No. 4,440,498 discloses a fiber optical gyroscope in which a 3.times.3-coupler is used for operation close to the quadrature point without using a modulator.
U.S. Pat. No. 4,479,715 discloses another fiber optical gyro equipped with a modified 3.times.3-coupler which derives the angular or rotational spin rate from the signals of the photo-diodes P.sub.1 and P.sub.2 in accordance with the relationship EQU (P.sub.1 -P.sub.2)/(P.sub.1 +P.sub.2)=f(.PHI.).
The advantage of this signal evaluation is seen in that the light intensity of the light source cancels itself so that fluctuations in the light intensity do not have any influence on the measured result.
The above described conventional fiber optical gyros and all other conventional gyros of this type, however, are subject to a number of problems outlined below. It is known that fiber optical gyros do not at all permit an operation with a reciprocal light path due to their construction required by the use of a 3.times.3-coupler. As a result, environmentally caused fluctuations of the transmission characteristics of the fiber optical coil are noticed in the form of a zero point drift and in a scale factor drift of the gyro signal. Such environmental fluctuations can, for example, be caused by temperature changes and variations in the pressure. The zero point errors can be avoided by using non-polarized light. However, the rather disturbing contrast variations of the interference signal, which variations cause said scale factor drift, remain because they are unavoidable. A precise analysis of the output signal shows for the output signals P.sub.2 and P.sub.3 of the detector photo-diodes D.sub.2 and D.sub.3 the following: EQU P.sub.2 =v.multidot.(A+kB.multidot.cos(.PHI.-c)).multidot.I EQU P.sub.3 =v.multidot.(A+kB.multidot.cos(.PHI.+c)).multidot.I,
when nonpolarized light is used. In the foregoing relationships the spin rate .OMEGA. and the Sagnac-Phase .PHI. are correlated with the scale factor s as follows: EQU .PHI.=s.multidot..OMEGA..
Further, "v" represents a conversion factor for describing the efficiency of the photo-diodes and "I" is the intensity of the light source LQ. The constants "A", "B", and "c" are coefficients which result from the transmission characteristics of the 3.times.3-coupler.
However, the contrast factor "k" in the above relationship is variable. The contrast factor "k" depends on the magnitude of polarization coupling centers which are always present in glass fibers. Such polarization coupling centers are very sensitive to temperature and pressure changes. Thus, the respective value of "k" is unknown and hence falsifies the evaluation of the Sagnac-Phase .PHI. which needs to be found. As a result, the gyro or spin rate .OMEGA. is also falsified. For example, the evaluation method disclosed in U.S. Pat. No. 4,479,715 would have the following result: ##EQU1## If the coupler coefficients A, B, and c are known and constant, the right side of the equation shows the Sagnac-Phase .PHI. that needs to be ascertained. However, the fluctuating unknown contrast factor "k" is also part of the right-hand side of the equation. Therefore, an exact determination of the spin rate .OMEGA. is not possible by the method of U.S. Pat. No. 4,479,715.
In order to obtain a sufficiently accurate result with conventional evaluating methods, it would have to be tried to keep the magnitude of the above mentioned polarization coupling centers as small as possible. This could be achieved by selecting very expensive specialized light conducting optical fibers and strongly damping embedding masses and so forth. Such features could keep the contract factor "k" substantially constant. However, the effort and expense for a still unsatisfactory result are very high.